Low tension igniter plugs for igniting fuel in gas engines have certain advantages over high tension igniter plugs, the foremost being increased system reliability since a significantly lower voltage is necessary to induce an electrical spark in low tension igniter plugs. However, low tension igniter plugs suffer a serious disadvantage over high tension igniter plugs, that being a much shorter useful life. Typically, conventional low tension igniter plugs maintain only approximately one-quarter to one-third of the life of high tension igniter plugs. As a result, the primary reason for not utilizing low tension systems on commercial engines has been the absence of a low tension igniter with a useful life equivalent to that of a high tension igniter.
In a conventional low tension ignition system, the useful life of the igniter plugs is generally limited by the amount of semiconductor material provided between the plug's central electrode and its outer metal electrode. Typically, the semiconductor material is coaxially disposed between the shell and the central conductor at the spark gap existing between the shell and the tip of the central conductor. In all types of igniter plugs, it is of considerable importance to maintain the spark at or very near the end of the igniter plug. However, in the conventional low tension igniter, the semiconductor element is permanently fixed in place. Consequently, as the semiconductor material nearest the tip is consumed, the surface of the semiconductor material recedes away from the tip such that the igniter eventually will fail to spark, or the spark will become recessed and fail to project into the combustion zone. Accordingly, this type of construction results in a limited useful life for the igniter plug.
Several techniques have been developed in the past for solving various problems associated with igniter plugs, but none have addressed the problems discussed above relating to the relatively short life of low tension igniter plugs. For example, U.S. Pat. No. 3,882,338 to Meyer discloses an igniter plug that includes a biasing arrangement acting upon an insulator separating the central electrode from the outer metal shell of the igniter plug. This biasing arrangement includes at least one spring which compensates for thermal contraction and expansion of the insulator so that the insulator is maintained in pressure contact with the surface of the outer electrode. This technique overcame problems associated with insulators that were rigidly mounted between the two electrodes often resulting in cracking of the insulator and eventual igniter failure due to thermal expansion and contraction. However, because the Meyer '338 patent is directed to high tension igniter plugs which do not utilize a semiconductor material, it does not address problems associated with consumption of the semiconductor material in low tension igniters resulting in variations in the spark plasma pattern and ultimately a shorter useful life.